The present invention relates to a fixing device for fixing a developer to a recording material and also relates to an image forming apparatus provided with such a fixing device.
An electrophotographic image forming apparatus includes a fixing device so as to fix and fuse a toner image transferred onto a sheet. In a conventional fixing device, a pressurizing roller is usually brought into pressure contact with a heating roller.
Recently, individual parts in image forming apparatuses have been improved to meet the needs of speeding up image formation (printing). A fixing device has been improved to increase a nip length of a fixing nip portion in order to provide an amount of heat necessary for a toner image to be fixed and fused onto a sheet conveyed at a high speed. For example, Japanese Patent Laid-Open Publication No. 2005-227463, as shown in FIG. 5, discloses a fixing device provided with: a heating roller 1001; a pressurizing roller 1002 that is pressed against the heating roller 1001; a belt stretching roller 1003 that is disposed on an upstream side in a paper conveying direction with respect to the pressurizing roller 1002 and is arranged on the pressurizing roller 1002 to be biased in the direction of an arrow G through a tension spring (not shown); a belt 1004 that is wrapped around the outer peripheries of the pressurizing roller 1002 and the belt stretch roller 1003 and moves while forming a fixing nip portion 1005 between the heating roller 1001 and the belt itself; and a preload spring (not shown) that biases the belt stretching roller 1003 in the direction of an arrow J and brings the belt stretch roller 1003 into contact with the heating roller 1001.
According to the fixing device described in Japanese Patent Laid-Open Publication No. 2005-227463, the belt 1004 is always stretched because the belt stretching roller 1003 is biased in the direction of the arrow G by the tension spring. For this reason, as the belt 1004 is extended in connection with a continuous use of the fixing device for a long period of time, the belt stretching roller 1003 moves to a position shown by a chain double-dashed line, and the nip length of the fixing nip portion 1005 changes (the nip length becomes longer in this case). As a result, a problem of exerting an adverse influence on toner fixation is caused.
In view of the above-stated problem, it is an object of the present invention to provide a fixing device capable of stably performing a fixing process without changing a nip length of a fixing nip portion and to provide an image forming apparatus including the fixing device.